For heat shrinkable films used for shrink packaging, shrink bundling and shrink label, for example, polyvinyl chloride (PVC), styrene-butadiene block copolymer (SBS), or polyester based resin have been used. Usually, a heat shrinkable film made of such plastic materials is supposed to be disposed of together with a product packaged by the heat shrinkable film after use of the product, in which case the heat shrinkable film poses a difficulty in incineration disposal or landfill disposal thereof. For instance, polyvinyl chloride discharges harmful gas during incineration disposal thereof. In case of landfill disposal, such plastic materials having high chemical stability are hardly biodegraded by the natural environment so that they may remain underground semipermanently. Thus, the disposal capacity of garbage landfilling facilities is reached in a short period of time. Further, when such plastic materials are disposed of in the natural environment, the scenery or the living environment of marine and other creatures are destroyed.
Consequentially, from a viewpoint of environmental protection, biodegradable materials have been much studied and developed in recent years. Polylactic acid, one of such biodegradable materials, is naturally hydrolyzed in soil or in water and harmlessly degraded by microorganisms. And, due to the low calorific value of combustion thereof, a combustor will not be damaged in the case of incineration disposal. Also, since the starting material of polylactic acid is derived from plant, it is not necessary to depend on oil which is an exhaustible resource.
For instance, a heat shrinkable film made of polylactic acid has been disclosed in Japanese Patent Application Publication No. Hei 5-212790. However, the disclosed heat shrinkable film has a high shrinkable temperature of 140 to 150° C. so that it cannot be used for other than a limited use.
In addition, a sheet or a film made of polylactic acid does not have a sufficient enough strength to put in practical use due to brittleness inherent to polylactic acid. Especially, a polylactic acid shrinkable film which is uniaxially stretched in remains brittle in unstretched directions, whereby it is easily cracked when impacted in the unstretched directions. Consequentially, in order to provide a breaking resistance to polylactic acid, it has been studied to blend polylactic acid-based resins with aliphatic polyester.
However, when a film made of polylactic acid-based resins blended with aliphatic polyester is stretched in a uniaxial direction, shrinkage in unstretched directions occurs. For instance, a heat shrinkable label, which used for a PET bottle, made of a film transversely-stretched shrinks not only in the transverse direction but also in a perpendicular direction (machinery direction) to the transverse direction (hereinafter referred to a longitudinal shrinkage), resulting in a bad appearance of the label. Therefore, a heat shrinkable film of polylactic acid-based resins stretched in a uniaxial direction and capable of preventing shrinkage in unstretched directions has been expected.